1. Field of the Invention
The present invention relates to a small-sized, long-lasting spark plug which is suitable for use in the internal combustion engines of automobiles and the like.
2. Description of the Related Art
Spark plugs used in automobile gasoline engines have conventionally consisted of, as shown in FIG. 1, an insulator 2 which covers a center electrode 3 made of nickel alloy, a fitting piece 1 fitted around the periphery of the insulator 2, a grounding electrode 4 provided at the end of the fitting piece 1 and a noble metal tip 52 provided at the tip of the grounding electrode 4, and it is designed so that a spark current flows in the discharge gap 6 between the noble metal tip 52 and the center electrode 1, thus igniting a gas mixture compressed in a combustion chamber by the electrical discharge generated thereby.
However, when the center electrode 1 consists of a nickel alloy alone, the wear rate is exceedingly high, rendering it difficult to provide a long-lasting spark plug.
Thus, long-lasting spark plugs have been produced by welding a platinum tip 51 as a noble metal material to the discharge section of the electrode made of the nickel alloy.
Nevertheless, internal combustion engines in recent years have larger valve sizes or greater numbers of valves required for greater output and mileage, and this has reduced the amount of space available for mounting of the spark plugs, thus leading to requirements for smaller sized spark plugs. Specifically, spark plugs used in motor bicycles and compact automobiles must have plug screw diameters of about 12 mm or smaller.
In the wake of these circumstances, it has been necessary to produce small spark plugs wherein the electrodes are welded with noble metal tips made of iridium and iridium alloy materials to provide superior wear resistance.
Japanese Unexamined Patent Publication No. 2-49388 discloses a spark plug which employs a noble metal tip 51 made of Ir or Ir alloy materials, which have greater wear resistance than Pt or Pt alloys.
Here, the present inventors have attempted to form a small-sized, wear-resistant noble metal tip 51 using Ir and Ir alloy materials, which noble metal tip 51 is applied for use in small-sized spark plugs.
However, the present inventors have experimentally confirmed that long-lasting, small-sized spark plugs provided with noble metal tips 51 made of Ir or Ir alloy materials frequently generate "side sparks" which fly in the gas volume 7 between the inner wall of the fitting piece 1 and the outer wall or housing of the insulator 2 during use, and these side sparks drastically reduce the driving characteristics such as idle stability and acceleration.
Consequently, it has been an object of the present invention to elucidate the reason for frequent generation of side sparks occurring with noble metal tips made of Ir or Ir alloys, and to provide a spark plug which greatly minimizes generation of side sparks despite being a small-sized spark plug employing iridium or iridium alloys.